Because the fuel cells have high energy conversion efficiency from a fuel to electricity without emitting toxic materials, they are attracting much attention as the next-generation power generators. Particularly ion-exchange-polymer-membrane-type fuel cells operable at temperatures of 150° C. or lower are recently vigorously investigated, with the expectation that they will be put into practical use in several years. Because this type of fuel cells are operable at relatively low temperatures, generate high power density, and can be reduced in size, they are suitable for automobiles and home installments, etc.
The ion-exchange-polymer-membrane-type fuel cell is usually constituted by unit cells each comprising an ion-exchange fluororesin membrane containing sulfonic acid groups as a solid electrolyte membrane, and a fuel electrode and an oxygen (air) electrode attached to both surfaces of this membrane. Each electrode is usually composed of a porous material comprising carbon black, in which a water-repellent tetrafluoroethylene (PTFE) resin and fine precious metal particles as a catalyst are dispersed. The unit cells are laminated via planar separators each having gas-flowing grooves for uniformly supplying a fuel gas and air on both surfaces, to constitute a fuel cell stack.
The separator is required to have not only electric conductivity and gas tightness, but also chemical and electrochemical stability (corrosion resistance). Accordingly, conventional fuel cell separators are mostly made of carbon materials such as graphite. Though graphite has low electric resistance and high corrosion resistance, it has low mechanical strength and suffers from high working cost. Particularly in the case of vehicle-mounted fuel cells, the separator materials are required to have high mechanical strength, making it difficult to use graphite separators.
Proposed recently is a graphite separator produced by injection-molding a mixture of graphite powder and a resin and baking it at a high temperature. However, the resin-containing graphite separator is poorer in mechanical strength, electric resistance and thermal conductivity than the metal, and higher in electric resistance than a separator consisting of only a graphite material. Accordingly, the resin-containing graphite separator suffers from the reduction of cell power, and temperature elevation, not suitable for use in high-power fuel cells. Also, because the resin-containing graphite separator is produced by high-temperature baking, it has a low density. Although the graphite separator can be impregnated with a resin after baking and then baked again to increase its density, it needs a complicated production steps.
Other than carbon material separators, separators having metal substrates are investigated. The metal separators have much higher electric resistance, gas tightness and mechanical strength than carbon separators. In addition, the use of a metal can make the separators thinner, advantageous in reducing the weight. However, the metal is more vulnerable to corrosion than the carbon materials, and when metal ions generated by corrosion enter into the electrolytic membrane, the ion conductivity of the membrane is lowered, adversely affecting the performance of cells. Among metals other than precious metals, stainless steel has excellent corrosion resistance and workability, but it usually does not have sufficient electric conductivity because of a passive layer formed on the surface.
To solve the above-described problems of metal separators, for instance, JP 11-162478 A discloses a method of plating a precious metal on the entire surface of a metal separator to have improved corrosion resistance and electric conductivity. Despite high effects of improving corrosion resistance and electric conductivity, this method is not practical because a thick corrosion-resistant plating increases production cost. JP 2003-272649 A and JP 2003-272653 A disclose methods of forming an extremely thin gold plating on a metal separator to improve corrosion resistance. However, because a thin gold film has low strength, it is likely to be damaged when the separator is brought into contact with an electrode, causing corrosion in damaged portions. Thus, JP 2003-272671 A proposes a method of filling a soft conductor between an electrode and a metal separator. The soft conductor interposed between the electrode and the separator protects a corrosion-resistant layer of the separator, thereby improving the performance of a cell, but it increases the production cost because of the increased numbers of parts and assembling steps.
Among the stainless steel, particularly austenitic stainless steel has excellent corrosion resistance and workability, and separators comprising austenitic stainless steel substrates are proposed. For instance, JP 2002-151111 A and JP 2003-193206 A disclose austenitic stainless steel separators having metal carbides and/or borides exposed on the surface. These separators have reduced contact resistance with current collectors because of the exposed metal carbides and/or borides. However, because the types of metal carbides and borides are limited, the electric conductivity of these separators does not reach a sufficient level. Particularly, these separators do not fully use the advantages of a metal surface because of a passive layer formed on the exposed stainless steel surface, resulting in insufficient electric conductivity. In addition, the stainless steel having metal carbides and/or borides dispersed is not commonly available, requiring large investment on facilities to produce it.
Nitriding is known as a method for surface-treating stainless steel. For instance, JP 2003-113449 A describes that meta-stable austenitic stainless steel is turned to a separator material suitable for a fuel cell with improved surface strength and fatigue resistance, by gas-nitriding or salt bath-nitriding at a temperature of 300-650° C. However, JP 2003-113449 A is silent about the corrosion resistance of the separator material formed. In general, a nitride layer formed on a stainless steel surface has such low corrosion resistance that it is dissolved away by corrosion when used in a strong-acidity environment like fuel cell separators. It is thus likely that metal ions attach to polymer electrolyte membranes and electrodes, thereby lowering their performance, and that a passive layer of stainless steel is exposed, resulting in increased contact resistance. Thus, the performance of a fuel cell is deteriorated. As described above, even if an austenitic stainless steel substrate is used, low-cost separators having excellent electric conductivity and corrosion resistance have not been obtained yet.